Microsoft's Outlook and Outlook Express both support S/MIME to some degree. Thunderbird does as well. But none of those software installations go out and request a digital certificate that makes it useful. So users are left finding the feature on their own. Why couldn't these companies, and the I.S.P.s that provide these email clients automatically request the certificates?
And why don't any of the web based mail clients support S/MIME at all? I vaguely remember one that did, but they dropped the support after they were purchased and absorbed into Yahoo.
These products are pretty damn easy to use once the certificate identifying the users is installed. And it could take care of a whole host of problems if they were more qidely used. And I'm not talking about such things that politicians and e-commerce companies tout as benefits of digital signatures. I'm talking about really useful stuff.
You could block all unsecured email. Viola! No spam. Or if you did get spam, you could find out who the spammer is, or which organization vouched for the person.
And when 
jalajscion or Kurukafa claim they never wrote something, you would have a digital signature showing the words were unaltered.
I think those two reasons alone would be enough to spur I.S.P.s to better encourage the use of S/MIME digital certificates.